The various devices and device regions illustrated herein are for N-channel devices or the like. However, this is merely for convenience of explanation and not intended to be limiting and persons of skill in the art will understand that the principles taught herein apply to devices of either conductivity type. Accordingly, the identification of particular regions as N-type or P-type is merely by way of illustration and not limitation and opposite conductivity type regions may be substituted in order to form devices of opposite conduction type.
FIG. 1 is a simplified schematic cross-sectional view of RESURF device 20, according to the prior art. Device 20 comprises P-type substrate 24, N-type drift space 26 and P-type well 30. N++ contact region 28 is provided to drift space 26. N++ contact region 28 has electrode 29 and external terminal 31. P-well 30 has P++ body contact region 32 therein and, by way of example and not intended to be limiting, N++ source region 34 therein. Electrode 33 with external terminal 35 makes ohmic contact to P++ body contact region 32 and optional N++ source region 34. The breakdown voltage VBD of device 20 is measured between terminals 35 and 31. Gate 36 and gate electrode 37 are illustrated, merely by way of example for an N channel transistor, but this is not intended to be limiting.
FIGS. 2-3 show plots 40, 50 of how the breakdown voltage VDB of device 20 varies as a function of the doping Na(24) of P-type substrate 24 (FIG. 2) and as a function of the doping Nd(26) of N-type drift space 26 (FIG. 3). Referring now to FIGS. 1 and 2 together, trace 42 illustrates how changes in the doping of substrate region 24 cause the breakdown voltage of lateral junction 25 between N-type drift space 26 and P-type well region 30 to vary, and trace 44 illustrates how changes in the doping of substrate region 24 causes the breakdown voltage of vertical junction 27 between N-type drift space 26 and P-type substrate 24 to vary. In this example, N-type drift space 26 has a doping level of ˜2E16 cm−3. The breakdown voltage of device 20 is given by the combined results shown by heavier trace 46. This indicates that optimum (i.e., highest) breakdown voltage 48 is obtained for a P-type substrate doping level of about Na˜4E15 cm−3.
Referring now to FIGS. 1 and 3 together, trace 52 illustrates how changes in the doping of N-type drift space 26 causes the breakdown voltage of vertical junction 27 between N-type drift space 26 and P-type substrate 24 to vary, and trace 54 illustrates how changes in the doping of substrate region 24 causes the breakdown voltage of lateral junction 25 between N-type drift space 26 and P-type well region 30 to vary. In this example, P-type substrate 24 has a doping level of Na˜4E15 cm−3. The breakdown voltage of device 20 is given by the combined results shown by heavier trace 56. This indicates that optimum (i.e., highest) breakdown voltage 58 is obtained for an N-type drift space doping level in the range of about Nd(26)˜1−2E16 cm−3. Considering FIGS. 2 and 3 together, it is apparent that there is only one doping value Na(24) of P-type substrate 24 and a very narrow range of doping levels Nd(26) for N-type drift space 26, that will give optimum breakdown voltage 58, in this case about VBD˜105 volts. It is known that by simultaneously adjusting the size and/or doping density of P-well 30, N-type drift space 26 and P-type substrate 24, that the breakdown voltage can be made larger, but this usually adversely affects other device properties. For example, making N-type drift space 26 longer can increase the breakdown voltage but also increases the ON-resistance which is undesirable.
Accordingly, it is desirable to provide semiconductor devices, especially RESURF type semiconductor devices having improved breakdown voltage, without a corresponding degradation of other important device properties. In addition, it is desirable that the modified structure and method needed to obtain such improved performance be compatible with available semiconductor device and integrated circuit fabrication tools and methods. Other desirable features and characteristics of the invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.